River sediments act as both sinks and secondary sources of natural radioactivity, affecting water, soil, and the food chain, with industrial and anthropogenic activities further redistributing radionuclides. This study reports the first radiological assessment of 21 sediment samples from diverse land-use areas along the transboundary Surma River, Sylhet, Bangladesh. The activity concentrations of 226Ra, 232Th, and 40K, measured using an HPGe gamma-ray detector, ranged from 11 to 27, 19 to 50, and 279 to 351 Bq/kg, respectively. Compared with global reference values (30, 35, and 400 Bq/kg for 226Ra, 232Th, and 40K, respectively), all 226Ra and 40K concentrations were below the averages, whereas several 232Th values (44–50 Bq/kg) slightly exceeded the world average. The calculated radium equivalent activity (Raeq = 79.71 Bq/kg), absorbed dose rates (Dout = 38.66 nGy/hr and Din = 46.39 nGy/hr), hazard indices (Hex = 0.22 and Hin = 0.26), annual effective dose equivalent (E = 0.28 mSv/y), gamma level index (Iγ = 0.59), and excess lifetime cancer risk (ELCR = 0.17) were all found to be within safe limits. No 137Cs was detected, confirming the absence of artificial radionuclide contamination. Multivariate statistical integration of correlation, cluster, and principal component analyses distinctly traced 226Ra and 232Th to a common geogenic origin as the principal radiological source, whereas 40K exhibited an independent lithological signature with minimal hazard contribution. The relatively low levels are attributed to the basin’s alluvial nature and dilution from continuous fluvial processes. Overall, the findings indicate that Surma River sediments pose no significant radiological risk to human health or the environment, while providing valuable baseline data for future monitoring of transboundary river systems under expanding industrial activity.